1. Field of the Invention
The present invention relates to method and device that uses an array antenna to locate the near-field source of a signal that falls incident on the array antenna.
2. Description of the Related Art
FIG. 1 shows a uniform linear array ULA with elements 0 to 6 receiving a signal from a source in the far field of the array ULA. When a transmission source is in the far field, it is assumed to be an infinite distance from the array, so that the received signal has a plane wavefront PW. Because the wavefront is plane, the incident signal has the same incident angle xcex8farfield at each element. Direction of Arrival (DOA) estimation algorithms such as MUSIC and ESPIRIT perform DOA estimation with great accuracy when the transmission source is in the far field of the array. For example, U.S. Pat. No. No. 5,854,612 (based on foreign priority of Japanese Patent Application No. 9-042877) describes that it is a relatively easy task to obtain the angle of the incident signal by calculating the phase difference of the signal received at the different antenna elements.
However, in most indoor applications, such as Wireless LAN, the source is in the near field. As shown in FIG. 2, when the transmission source S in the near field of the reception array, propagation waves from the transmission source have a spherical wavefront SW. The arrival angle is different at each element, for example, angles xcex80, xcex84, and xcex86 at elements 0, 4 and 6, respectively. When the above-described DOA estimation algorithms are used for near field sources, the estimate of the source location, and consequentially the radiation diagram, can be distorted.
Attempts have been made to improve results of DOA estimation performed for near field sources using far field DOA estimation algorithms. For example Kennedy et al disclose such a method in xe2x80x9cBroadband Near field Beamforming Using a Radial Beam pattern Transformationxe2x80x9d, IEEE Trans. on Signal Proc., vol. 46, no 8, August 1998. However, this method requires a previous precise knowledge of the distance from the source to each element of the array.
Asano et al disclose another method in xe2x80x9cSource separation using subspace method and spatial inverse filterxe2x80x9d, IEICE Technical Report, EA 99-22, pp. 1-7, June 1996. This method estimates not only DOA, but also distance to the source. However, without some general knowledge of the source""s location, the distance to the source can be any value from 0 to infinity. Therefore, a range domain with essentially no bounds must be searched to find the source.
It is an objective of the present invention to overcome the above-described problems, and to provide a source location estimation device capable of estimating source location in the near field using far field DOA estimation algorithms, with only a limited domain search even when source location is completely unknown.
To achieve the above-described objective, a source location estimation device according to the present invention includes an array antenna, first and second samplers, a direction-of-arrival estimator, a source location estimator, and a sampling adjuster.
The array antenna includes two sub-arrays. Each sub-array has at least three elements with at least one uncommon element. The first sampler samples elements of one sub-array, and the second sampler samples elements of the other sub-array. The direction-of-arrival estimator uses samples from the samplers to make a separate direction-of-arrival estimate for each sub-array for direction of arrival of a signal from a source. The source location estimator estimates distances from the source to each element based on the separate direction-of-arrival estimates from the direction-of-arrival estimator. The sampling adjuster adjusts timing of sampling performed by the samplers based on the distances from the source location estimator.
With this configuration, the sampling timing of the sampler is synchronized to follow the sphericity of the incoming wavefront. The near field distortion is gradually removed with each iteration, so that the source is located with increasingly higher precision. No previous knowledge of the source location is needed, because only a simple angle-domain is searched, limited to the interval between 0 (0 degrees) and xcfx80 (180 degrees).
According to another aspect of the present invention, the sampling adjuster adjusts timing of sampling by the first sampler based on the following formula:             Δτ      "RightBracketingBar"        i          θ      0        =                    (                                            d              ^                        i                    -                                    d              0                        ^                          )            c        -                            i          ⁢                      xe2x80x83                    ⁢          λΔ          ⁢                      xe2x80x83                    ⁢          e                c            ⁢              cos        ⁡                  (                                    θ              0                        ^                    )                    
wherein i represents the target element of sampling from elements 0 to Lsxe2x88x921, element 0 being the optimum referential element of the one sub-array;
xcex94xcfx84|ixcex80 represents the error between the far-field delay and the near-field delay with respect to the element 0 and the target element;
{circumflex over (d)}i represents the distance between the source and the target element estimated by the source location estimator;
{circumflex over (d)}0 represents the distance between the source and the element 0 estimated by the source location estimator;
c represents the speed of light;
xcexxcex94e represents inter-element distance; and
{circumflex over (xcex8)}0 represents direction of arrival estimated for the element 0 of the one sub-array by the direction-of-arrival estimator.
Further, the sampling adjuster adjusts timing of sampling by the second sampler based on the following formula:             Δτ      "RightBracketingBar"        i          θ              L        -        1              =                    (                                            d              ^                        i                    -                                    d              0                        ^                          )            c        -                            i          ⁢                      xe2x80x83                    ⁢          λΔ          ⁢                      xe2x80x83                    ⁢          e                c            ⁢              cos        ⁡                  (                                    θ              ^                                      L              -              1                                )                    
wherein i represents the target element of sampling from elements L-Ls to L-1, the element L-1 being the optimum referential element of the other sub-array;
xcex94xcfx84|ixcex8L-1 represents the error between the far-field delay and the near-field delay with respect to the element L-1 and the target element; and
{circumflex over (xcex8)}L-1 represents the direction of arrival estimated for the element L-1 by the direction-of-arrival estimator.
With this configuration, the sampling adjuster can adjust timing of sampling for elements using a relatively simple algorithm.
According to another aspect of the present invention, the array antenna includes elements 0 to L-1 for a total of L elements. One sub-array includes elements 0 to Ls-1, wherein Ls less than L. Element 0 is an optimum referential element of the one sub-array. The other sub-array is obtained by a shift of L-Ls elements, and so includes elements L-Ls to L-1. The element L-1 is an optimum referential element of the other sub-array.
Further, the source location estimator estimates distances between the source and each of the elements 0 to L-1 based on the following formulas:                     d        ^            0        =                            (                      L            -            1                    )                ⁢        Δ        ⁢                  xe2x80x83                ⁢        e        ⁢                  xe2x80x83                ⁢        λ                    "LeftBracketingBar"                                                            sin                ⁢                                  xe2x80x83                                ⁢                                  (                                                            θ                      ^                                        0                                    )                                                            sin                ⁡                                  (                                                            θ                      ^                                                              L                      -                      1                                                        )                                                      ⁢                          cos              ⁡                              (                                                      θ                    ^                                                        L                    -                    1                                                  )                                              -                      cos            ⁡                          (                                                θ                  ^                                0                            )                                      "RightBracketingBar"              and                    d        ^                    i         greater than         0              =                                        (                                                            d                                      0                    ⁢                                          xe2x80x83                                                                      ⁢                                  cos                  ⁡                                      (                                                                  θ                        ^                                            0                                        )                                                              +                              i                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                e                ⁢                                  xe2x80x83                                ⁢                λ                                      )                    2                +                  (                                    d              0                        ⁢                                          sin                ⁡                                  (                                                            θ                      ^                                        0                                    )                                            2                                )                    
wherein i represents a target element of sampling from elements 0 to L-1;
{circumflex over (d)}0 represents the distance between the source and the element 0 estimated by the source location estimator;
xcexxcex94e represents the inter-element distance;
{circumflex over (xcex8)}0 represents the direction of arrival estimated for the element 0 by the direction-of-arrival estimator:
{circumflex over (xcex8)}L-1 represents the direction of arrival estimated for the element L-1 by the direction-of-arrival estimator; and
{circumflex over (d)}i represents the distance between the source and the target element estimated by the source location estimator.
With this configuration, the source location estimator can estimate distances between the source and each of the elements 0 to L-1 using a relatively simple algorithm.
According to still another aspect of the present invention, a beamformer is provided that performs beamforming based on the distances estimated by the source location estimator. The beamformer can track the movement of a mobile terminal while also steering beamforming that is performed at the mobile terminal. Both null steering and beam steering can be accurately performed.
A method according to the present invention includes the steps of sampling elements of two sub-arrays having at least three elements each, wherein at least one element of each sub-array is not shared with the other sub-array; using sampling results to make a separate direction-of-arrival estimate for each sub-array for direction of arrival of a signal from a source; estimating distances from the source to each element based on the separate direction-of-arrival estimates; and adjusting timing of sampling of the elements based on the distances.
With this method, the sampling timing of the sampler is synchronized to follow the sphericity of the incoming wavefront. The near field distortion is gradually removed with each iteration, so that the source is located with increasingly higher precision. No previous knowledge of the source location is needed, because only a simple angle-domain is searched, limited to the interval between 0 (0 degrees) and xcfx80 (180 degrees).